March 8th, 2017, 12:02 PM  #1 
Newbie Joined: Mar 2017 From: uk Posts: 1 Thanks: 0  Solve trigonometry problem
Hello, Please help. I have been given the question: Find all solutions (in degrees) in the range âˆ’360Â° < Î¸ < 360Â° such that cos(4Î¸) = cos(Î¸) I have no clue how to even approach this question, can someone please help me? Thanks in advance. Last edited by skipjack; March 8th, 2017 at 12:22 PM. 
March 8th, 2017, 12:32 PM  #2 
Global Moderator Joined: Dec 2006 Posts: 16,595 Thanks: 1199 
The equation implies that 4Î¸ = Â±Î¸ + k*360Â°, where k is an integer. Hence Î¸ is an integer multiple of 72Â° or 120Â°. List the 13 solutions that lie in the specified interval. 
March 8th, 2017, 03:56 PM  #3  
Member Joined: Oct 2016 From: Melbourne Posts: 77 Thanks: 35  Quote:
This factors further. Hint: $\displaystyle \begin{align*} \cos{(x)} = \frac{1}{2} \end{align*}$ is another solution.  

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